LESSONS LEARNED WHILE MEASURING FUEL SYSTEM DIFFERENTIAL PRESSURE MARK HEATON AIR FORCE FLIGHT TEST CENTER EDWARDS AFB, CA 10 MAY 2011

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1 AFFTC-PA LESSONS LEARNED WHILE MEASURING FUEL SYSTEM DIFFERENTIAL PRESSURE A F F T C m MARK HEATON AIR FORCE FLIGHT TEST CENTER EDWARDS AFB, CA 10 MAY 2011 Approved for public release A: distribution is unlimited. AIR FORCE FLIGHT TEST CENTER EDWARDS AIR FORCE BASE, CALIFORNIA AIR FORCE MATERIEL COMMAND UNITED STATES AIR FORCE

2 REPORT DOCUMENTATION PAGE Form Approved OMB No Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports ( ), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YYYY) TITLE AND SUBTITLE 2. REPORT TYPE TECHNICAL PRESENTATION 3. DATES COVERED (From - To) 5a. CONTRACT NUMBER Lessons learned while measuring fuel system differential pressure 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) Mark Heaton 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT NUMBER AFFTC-PA SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) AFFTC 412 Test Wing ENI Edwards AFB, CA 10. SPONSOR/MONITOR S ACRONYM(S) N/A 11. SPONSOR/MONITOR S REPORT NUMBER(S) 12. DISTRIBUTION / AVAILABILITY STATEMENT Approved for public release A: distribution is unlimited. 13. SUPPLEMENTARY NOTES CA: Air Force Flight Test Center Edwards AFB CA CC: ABSTRACT A recent aircraft project required the measurement of differential pressures across a fuel filter during engine operation. Early testing showed intermittent noise in the system. Additionally, transducers were failing. Attempts to troubleshoot the problem led to the discovery of unexpected large pressure transients within the fuel system which were causing the noise and damaging the transducers. Attempts were made, with some success, to gather the data without extensively modifying the existing system. This presentation provides a brief review of what was done and what was learned from this program. 15. SUBJECT TERMS Fuel Pressure Transducer, Differential Pressure Transducer, Pressure Noise, Data Acquisition System (DAS) 16. SECURITY CLASSIFICATION OF: Unclassified a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 17. LIMITATION OF ABSTRACT None 18. NUMBER OF PAGES 19a. NAME OF RESPONSIBLE PERSON 412 TENG/EN (Tech Pubs) 19b. TELEPHONE NUMBER (include area code) Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. Z39.18

3 Air Force Flight Test Center War-Winning Capabilities On Time, On Cost Lessons learned while measuring fuel system differential pressure 10 May 2011 Approved for public release; distribution is unlimited. AFFTC-PA No.: Mark Heaton 412TENG/ENI I n t e g r i t y - S e r v i c e - E x c e l l e n c e 1

4 Requirement Method Early problems Resolution Results Lessons learned Overview 2

5 Program Requirement Demonstrate fuel pressure characteristics Ice buildup on strainer In flight Normally: 0.2 psid Clean filter Strainer has bypass valve Opens when filter clogs Opens at ~ 1 psid Full flow at 2.1 psid Blocked strainer 3

6 Approaches (1) Differential Pressure across strainer Engines already instrumented Video of strainer in housing Intrusion of camera or borescope in fuel Extensive modification to strainer housing Would ice be obvious? Require further lab testing Time 4

7 Approaches (2) Use Hall Effect Sensor on bypass valve Require modification to strainer housing Provide actual bypass valve position Seal bypass valve Method used during lab testing Minor modification to strainer housing Potential engine flameout 5

8 Design Measure differential pressure across strainer +/- 15psid transducer Amplified, 5V output Absolute pressure measured on strainer output 100psia transducer Passive bridge, 100mV output Used transducers on hand 6

9 Layout strainer Inlet Differential Transducer Fuel strainer Absolute Transducer 412TENG/ENIM P07A017A 7

10 Initial Testing Data looked good during acceptance testing Later ground testing noisy Replaced transducers to combat noise 8

11 Troubleshooting Difficulties Priority to test Plane available for limited time Try quick simple fixes Access to aircraft Other programs on aircraft Slow update rate from onboard display Hard to quantify noise levels Data access Fuel system knowledge 9

12 Baseline flight First Flight New transducer Day prior ground test good 10

13 What To Do? Noise or Data? Quick fixes Add dummy transducer Perform autopsy on failed transducer Plan for alternatives 11

14 Noise or Data? Indications point to fluctuations being data Hard transducer failure No significant noise on other channels Noise levels change with throttle change Noise voltage levels Absolute transducer-passive, 5mv noise seen Differential transducer-active, 2v noise seen Similar EU values between the two Lab tests showed no pressure fluctuations Flight representative? 12

15 Quick Fixes Replaced transducers Passive vs. active DAS-Transducer coupling? Little or no effect Altered grounding of transducer and DAS Little or no effect Instrument for temperature Not a factor, < 50 C 13

16 Dummy Transducer Thermocouple Ground Strap Dummy transducer Installed on another engine Previously instrumented Transducer removed Not connected to fuel Line 412TENG/ENIM P07A017A Also noisy Coincidence? Bad transducer? Noisy ground? Noise Level 14

17 Transducer Autopsy Electronics still good Amplified output at -1.0V Zero adjustment worked Could not be brought to zero Not responsive to pressure Diaphragm distorted Bridge de-bonded & broken Causes Overpressure Severe dynamic fluctuations Boeing Failure Analysis 5 Nov 2010 Damaged Areas 15

18 Cause Fuel pump pressure noise Cavitation Surge Valves Water Hammer effect Affect each side of transducer differently Strainer Different paths Air pockets 16

19 Fuel system Investigate fuel system Boost pumps Low pressure Normally fuel comes from boost pumps Auxiliary pumps High pressure Test fuel comes from auxiliary pumps Significant pressure fluctuations observed Switching pumps often caused noise Accompanied by pressure spikes 100+psi Became obvious later 17

20 New plan Add redundancy Use two absolute pressure transducers Input and output of strainer Lower fidelity Good results when filtered Alternative Instrumentation ruled out Hall effect sensor Seal bypass valve shut Add pressure snubbers Porous membrane to dampen transients Applied to differential transducer only 18

21 Layout Strainer Outlet Strainer Inlet Strainer Differential 412TENG/ENIM P07A017A Snubber 19

22 Initial Ground Test 20

23 Later Ground Test Boost pumps Gravity feed Aux pumps Pressure spikes Pressure noise 21

24 First Flight Boost pumps Aux pumps Pressure noise Drift and Bias 22

25 Last flight 23 Transducer Failure Bias

26 Lessons Learned (1) Easy to look for problems with instrumentation Spent to much time looking for noise Coincidence wreaks havoc Fuel pumps Dummy transducer Know the system Did not expect high transients or fluctuations Did not understand fuel pump differences 24

27 Lessons Learned (2) Look at the data Onboard display limited Slow turnaround of data System characteristics in data Fuel systems may exhibit transient pressures Valves and pumps 100+psia and 30psid transients Redundancy is good Second absolute transducer saved last flight Lab test may not represent flight No transients observed in lab 25

28 Summary Questions? 26

29 Back Up Slides 27

30 Last flight 28

31 Dummy Noise Connected Reacting to throttle Lower level Scale Coincidence? Bad transducer? Noisy ground? Note Scale Dummy 29

32 Second Ground Test 30

33 Engine Start Transients 31

34 Later Ground Test 32

35 Flight 2 Flight test after modification Data filtered Start of test 33